Multi-Step Electrodeposition Technique for Hierarchical Porous Coatings with Tunable Wickability, Wettability and Durability

ABSTRACT

Porous metal coatings are produced by multi-step electrodeposition method involving application of higher current density at lower duration and lower current density for longer durations. During higher current step, the evolved hydrogen bubbles serve as the self-collapsing templates to deposit the metal ions and during lower current density at longer duration step, the coating is applied to improve the bond strength. The multi-step depositions by alternating these steps provides the development of more complex porous structures that can have desired interconnectivity between the pores and have desired morphology, and desired wettability and wickability characteristics, and have a good bond strength between the coated layers as well as coated layers and the base substrate.

CROSS REFERENCE

This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 62/620,652, filed Jan. 23, 2018, which is hereby incorporated by reference in its entirety.

This invention was made with government support under grant number 1335927 awarded by the National Science Foundation. The government has certain rights in this invention.

FIELD

This invention relates to a multi-step electrodeposition technique, and in particular a multi-step electrodeposition technique for hierarchical porous coatings having tunable wickability, wettability and durability of the coated material.

BACKGROUND

Electrochemical deposition is the process of coating solids on conductive base materials to modify the surface properties of the base material. An electrochemical cell is typically composed of an electrolyte comprised of positive and negative ions dissolved in water or a solvent, usually prepared from metal salts, and two working electrodes that can be of either a conducting or semiconducting nature known as a cathode (on which the coating is desired) and an anode. The applied electric current (rate of the motion of the electric charge) between the two electrodes under an external voltage causes the migration and diffusion of the positive and negative ions, also known as the charged species. An electric voltage is the electric force per unit charge. The direction of the field is the direction of the force exerted on the positive charge and from the positive charge towards the negative charge.

Traditionally, electrodeposition has been performed using a single-step or a two-step method that involves varying current, voltage and duration. The bonding of these deposited coatings on the base material is found to be unsustainable and unfit for prolonged usage. Such electrodeposition techniques are also limited by their control over the resultant morphological features, such as porosity and thickness, and consequential surface properties. Advantages of the electrodeposition method over other coating processes include: occurrence at ambient temperature and pressure; controlled deposition for tunable coating thickness and porosity; ability to coat substrates of varying shapes and thickness; and cost effectiveness.

The principle behind electrodeposition is the use of an electric current to strip and transfer the cations from a material (electrode) in the electrolytic solution and coat the cations/material in the form of a thin film or discrete electrodeposits onto a substrate with the help of a conductive surface (the second electrode). The electrodes are placed in an electrolyte solution that contains both positively charged ions, i.e., cations and negatively charged ions, i.e., anions. When an external electric field is applied, the cations depart to the cathode and get deposited as metal. On the surface of the electrodes, conduction of ions to electrons takes place as a result of the electrochemical reaction (or the redox reactions) that occurs between the chemical species and the electrode.

The reduction reactions occur at the substrate electrode (which is to be coated) that involves reduction of metal ions as a result of accepting the electrons from the electrode and the oxidation reaction for most counter electrodes in case of inert counter electrodes placed in the aqueous electrolytes the reaction that occurs is the electrolysis of the water producing hydrogen and oxygen gases which completes the electrical circuit. The hydrogen gas released as a result of hydrogen evolution reaction (HER) assists in the electrodeposition process. The copper ions released during the electrochemical reactions surround the hydrogen gas bubbles that serve as the template for open porous network coatings.

SUMMARY

This invention presents an electrochemical deposition method involving multiple steps of high and low current application for depositing coating metal in hierarchical porous fashion.

In accordance with one aspect of the present disclosure, there is provided a multi-step electrodeposition method including: providing an electrolyte including positive and negative ions; placing an anode electrode and a metal substrate cathode electrode in the electrolyte; and implementing a multi-step process between the anode electrode and the cathode electrode, wherein the multi-step process includes three or more steps of alternating a low current density step sufficient to produce deposition of a metal layer but insufficient to produce dynamic templating deposition of metal on the substrate and a high current density step sufficient to produce dynamic templating deposition of metal on the substrate.

In accordance with another aspect of the present disclosure, the multi-step electrodeposition method includes:

a first step including a low current density for long duration without evolution of hydrogen bubbles supporting dynamic templating deposition to provide a base layer to support the following deposition;

a second step including a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles and dynamic templating deposition; and

a third step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

In accordance with another aspect of the present disclosure, the multi-step method further includes:

a fourth step including a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and

a fifth step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

In accordance with another aspect of the present disclosure, the multi-step method further includes:

a sixth step including a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and

a seventh step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

In accordance with another aspect of the present disclosure, the multi-step electrodeposition method includes:

a first step including a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition;

a second step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition;

a third step including a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition; and

a fourth step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

In accordance with another aspect of the present disclosure, the multi-step method further comprises:

a fifth step including a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition; and

a sixth step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

In accordance with another aspect of the present disclosure, the multi-step method further includes:

a seventh step including a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and

an eighth step including a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.

These and other aspects of the present disclosure will become apparent upon a review of the following detailed description and the claims appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of an electrolytic cell and FIG. 1B is a schematic representation of a hydrogen gas bubble assisted dynamic templated metal electrodeposition process in accordance with an embodiment of the present invention;

FIG. 2A is a schematic representation showing 3 steps of a multi-step electrodeposition method and FIG. 2B is a schematic representation showing a fourth and fifth step of the multi-step electrodeposition method of FIG. 2A in accordance with an embodiment of the present invention;

FIG. 3A is a schematic representation showing 4 steps of a multi-step electrodeposition method and FIG. 3B is a schematic representation showing a fifth and sixth step of the multi-step electrodeposition method of FIG. 3A in accordance with an embodiment of the present invention;

FIGS. 4A-C show a plain copper chip (FIG. 4A) 3D view, (FIG. 4B) front view, and (FIG. 4C) top view;

FIG. 5 shows a delineated working area on a test surface;

FIG. 6 shows a schematic of an electrochemical cell;

FIG. 7 shows a schematic of a pool boiling test system;

FIG. 8 shows a schematic of a heater assembly and data acquisition;

FIG. 9 is a plot showing uncertainty in heat flux for a Six Step (6-S) Cu on Cu electrodeposited chip;

FIGS. 10A-D is a set of Scanning Electron Microscope (SEM) images of the copper on copper two and six step electrodeposited chips using a Galvanostatic method, (FIG. 10A) 70° tilt, 2-S, (FIG. 10B) 70° tilt, 6-S, (FIG. 10C) top view, 2-S, and (FIG. 10D) top view, 6-S;

FIGS. 11A-B is a set of SEM images of top view of copper on copper electrodeposited chip showing the morphology at 5 μm with 10 kX magnification (FIG. 11A) 2-S, (FIG. 11B) 6-S;

FIGS. 12A-D is a set of Scanning Electron Microscope (SEM) images of the copper on copper two and three step electrodeposited chips (FIG. 12A) & (FIG. 12C) top view 2-S, (FIG. 12B) & (FIG. 12D) top view, 3-S;

FIGS. 13A-D is a set of Scanning Electron Microscope (SEM) images of the copper on copper two and seven step electrodeposited chips (FIG. 13A) & (FIG. 13C) Top view 2-S, (FIG. 13B) & (FIG. 13D) top view, 7-S;

FIG. 14 is an X-Ray Diffraction of Cu on Cu electrodeposited chips;

FIG. 15 is a plot showing a comparison of contact angle change over time of copper on copper electrodeposited chips, Two Step (2-S) and Six Step (6-S);

FIG. 16 is a plot showing change of droplet volume over time of copper on copper 2-S and 6-S electrodeposited chips;

FIG. 17 is a plot showing a comparison of contact angle change over time of copper on copper electrodeposited chips, Two Step (2-S) and Seven Step (7-S);

FIG. 18 is a plot showing change of droplet volume over time of copper on copper 2-S and 6-S electrodeposited chips;

FIG. 19 is a schematic of top view of copper chip with Kapton® tape on the sides;

FIG. 20 is a plot showing a comparison of heat flux of plain copper chip, two-step (2-S), three-step (3-S), six-step (6-S), and seven-step (7-S) electrodeposited copper on copper chip;

FIG. 21 is a plot showing a comparison of heat transfer performance of plain copper chip, two-step (2-S), three-step (3-S), six-step (6-S), and seven-step (7-S) electrodeposited copper on copper chip;

FIGS. 22A-B is a picture of a test surface of aged (FIG. 22A) Two-Step (2-S), (FIG. 22B) Six-Step (6-S) copper on copper electrodeposited chips;

FIGS. 23A-B is a set of Laser Confocal Microscope images of aged (FIG. 23A) Two-Step (2-S), and (FIG. 23B) Six-Step (6-S) copper on copper electrodeposited chips;

FIG. 24 is a plot showing a comparison of wall superheat of 2-S copper on copper deposited chip at different repetitive pool boiling tests (R1—first repetitive test);

FIG. 25 is a plot showing a comparison of wall superheat of 6-S copper on copper deposited chip at different repetitive pool boiling tests (R1—first repetitive test);

FIG. 26 is a plot showing a heat flux comparison of aged 2-S and 6-S copper on copper electrodeposited chips;

FIG. 27 is a plot showing a comparison of heat transfer coefficient of plain copper chip, aged 2-S and 6-S copper on copper deposited chips;

FIG. 28 is a plot showing a comparison of wall superheat of 3-S copper on copper deposited chip at different repetitive pool boiling tests (R1—first repetitive test);

FIG. 29 is a plot showing a comparison of wall superheat of 7-S copper on copper deposited chip at different repetitive pool boiling tests (R1—first repetitive test); and

FIG. 30 is a plot showing a comparison of wall superheat of 2-S (˜117 μm) copper on copper deposited chip at different repetitive pool boiling tests (R1—first repetitive test).

DETAILED DESCRIPTION

The present invention relates to a multi-step electrodeposition technique which enhances the surface properties and bonding between the deposited materials. In an embodiment, the multi-step deposition includes 3 or more steps.

The term “dynamic templating” refers to building up of the coating material on the substrate around the hydrogen bubbles to form open porous network structures of the coating material on the substrate.

The term “high current” (HC) refers to the applied current density that is regarded high if it aids in the flow of sufficient hydrogen ions to create hydrogen bubbles at the electrode/metal-solution interface for dynamic templating, and metal ions that can be deposited surrounding the bubbles. The term “higher current” is also used here to differentiate between the high and low current conditions.

The term “low current” (LC) refers to the applied current that is not sufficient to generate enough hydrogen ions to produce hydrogen gas bubbles for dynamic templating process but generates some metal ions that get bonded around the bubble-templated coatings obtained in the previous high current step. The term “lower current” is also used to differentiate between the high and low current conditions.

The term “short duration” (LT) refers to the deposition time during which high current is applied that results in the evolution of hydrogen gas bubbles that provide a framework for the dynamic templating of the metal ions to generate compact porous deposited features. The term “shorter duration” is also used to differentiate between the short and long duration conditions.

The term “long duration” (HT) refers to duration of application of low current that does not produce hydrogen bubbles to support dynamic templating process. The fewer metal ions produced during the electrochemical reaction as a result of low current get deposited on and around the porous network of metal ions that was produced in the short duration (LT) step. The term “longer duration” is also used to differentiate between the short and long duration conditions.

The term “evolution of H₂ bubbles” refers to Electrochemical Hydrogen Evolution Reaction (HER) which is a surface reaction in which the hydrogen ions at the electrode/solution interface combine with protons from the electrolyte to form hydrogen protons and eventually hydrogen gas bubbles. This occurs due to the flow of electrons as a result of the applied current.

The term “evolution of insufficient H₂ bubbles” is used in reference to formation of hydrogen bubbles which is dependent on the composition of electrolyte and the electrode metal, metal type, metal/hydrogen interaction, and applied current in an insufficient amount to support dynamic templating. An inadequate amount of ions in the electrolyte solution leads to a lack of enough bubbles to support the dynamic templating process.

FIGS. 1A and 1B represent an embodiment of the process of application of higher and lower current densities applied alternately to deposit the coating metal ions around the hydrogen bubbles 18 and to strengthen the coatings 4 on the substrate. An electrochemical reaction occurs in the electrolyte 11 due to the applied high current density (which flows through insulated copper wires 14) for a shorter duration which results in the generation of hydrogen gas bubbles 18. The copper particles 10 from anode 9 get deposited around the bubbles on substrate (cathode 15) forming an open network/porous morphology of the copper particles. This is followed by a longer duration low current density application that causes the deposited copper particles to bind and form a layer of electrodeposited copper 4. This is depicted in FIGS. 1A and 1B. This is comparable to the process of placements of brick, with some mortar, and application of additional mortar around the joints.

FIGS. 2 and 3 represent embodiments of the invention showing multi-step electrodeposition processes. FIG. 2A shows a 3 step process which includes an initial step of application of low current density that results in deposition of an initial base layer 12 of the coating material on the substrate 16. This layer acts as a support to sustain the deposition of additional coating material 20 with enhanced bonding. This initial step is helpful because the following step involves using a higher current density to deposit copper particles 10 in an open porous network and the base layer provides a suitable bonding surface. The last step of low current density in FIG. 2A ensures the strong adhesion of the copper particles with the base layer 21, providing a unique microporous structure with higher porosity. In accordance with an embodiment of the present invention, FIG. 2B shows a fourth and a fifth step of the multi-step electrodeposition process of FIG. 2A. As indicated in FIG. 2B, the final surface achieved after third step of electrodeposition (i) (in FIG. 2A), is considered as is for the fourth and fifth step of electrodeposition. After supplying a higher current density in the fourth step, copper particles from high current step 19 deposit around the hydrogen bubbles on the existing layer (i). The fifth step of low current density again ensures the strong adhesion of the newly deposited copper particles 28 with the deposited layer and the substrate. In an embodiment, a sixth step of high current density and a seventh step of low current density can follow the fifth step (not shown). The multi-step electrodeposition method basically includes alternating steps of high and low current density with short and long duration respectively, that control the hydrogen evolution, and metal nucleation and deposition. Using three or more steps provides a better control on the morphology of the coating while improving bond strength.

FIG. 3A shows a schematic of a 4 step electrodeposition process with alternate high and low current density supplied for short and long duration, respectively. After supplying a higher current density, adhesion and the bond strength of the deposited copper layer 22 is strengthened using a low current density step. To achieve further enhancement in bond strength and porosity, a high current density step is added which deposits a new layer of copper particles 27 on the existing layer. This new layer is strengthened again by adding a fourth step of lower current density. A small amount of copper deposition 8 in the fourth step increases the bond strength of the coating. FIG. 3B shows a fifth and a sixth step of the multi-step electrodeposition process of FIG. 3A. As indicated in FIG. 3B, the final surface achieved after fourth step of electrodeposition i.e. (ii) (in FIG. 3A), is considered as is for the fifth and sixth step of electrodeposition. When a higher current density is supplied in the fifth step, copper particles from high current step 19 deposit around hydrogen bubbles on the existing layer achieved by four steps (ii). The sixth step of lower current density improves the adhesion of the copper particles by creating a concrete layer 24 around deposited copper particles. This boosts the bond strength and a distinctive structure with a very high porosity is achieved. In an embodiment, a seventh step of high current and an eighth step of low current can follow the sixth step (not shown). Overall, the resultant morphology depends on, among other things: applied current, electrolyte composition, deposition time, surface substructures, temperature, electrode properties, number of steps, and duration during each step.

The hydrogen evolution reaction rate depends on the applied current density which in turn is dependent on the depositing metal-hydrogen chemisorption energy or the dissociation of hydrogen ions from the electrode surface that further combines with protons to form hydrogen gas. This is dependent on the exchange current density which is defined as the rate of hydrogen evolution per surface area at the electrode. This exchange current density varies for different metals which is readily available in the literature. Metals that have weak interaction energy with hydrogen do not abet in passage of sufficient electrons, whereas, metals that interact strongly with hydrogen result in greater adherence on to the surface and not getting released in the solution instantly.

The applied current density and the duration control the surface morphology characteristics such as porosity and thickness, and the subsequent surface properties such as wickability, wettability, contact angle, hydrophilicity, and hydrophobicity of the electrodeposited coatings. Bubble behavior dictates the size of the pores. Higher bubble generation rate leads to shorter residence times that further control the coalescence of the bubbles. Reduced coalescence results in the smaller pore sizes of the deposited materials.

Effectively, higher current density results in production of higher amount of hydrogen ions that result in formation of hydrogen bubbles that provide the anatomy for the porous network and subsequent higher amount of metal ions produced in this step get deposited around these hydrogen bubbles as explained previously in FIGS. 3A and 3B. The multi-step electrochemical method comprises these alternate high and low current steps. A distinct porous and durable coating can be achieved by either starting with a low current step that results in the formation of an initial base layer of deposited metal that supports a bonding ground for the successive deposited metal ions (seen in FIGS. 2A and 2B), or starting with a high current step that adopts hydrogen bubbles to provide a framework for the deposition that gets strengthened by the subsequent low current step (seen in FIGS. 3A and 3B). However, when the multiple electrodeposition steps are terminated at the high current step, the deposited layer will constitute large amounts of deposited metal that may not necessarily be strong.

Dynamic Hydrogen Bubble Template (DHBT) or dynamic templating is a method to create a porous structure on the substrate. The electrochemical reaction takes place when direct current is supplied through the electrochemical cell. First, at a higher current density supply, hydrogen bubbles are formed on the cathode (substrate). Typically, the electrolysis of water in the electrolyte creates the hydrogen gas. If the evolution of bubbles is continued, the copper ions start to grow within the interstitial spaces between hydrogen bubbles. The resultant hydrogen bubbles behave as a dynamic template around which copper particles deposit and grow. When the higher current density supply is stopped, the hydrogen gas bubbles collapse leaving behind the porous open network of copper. The size of the pores is determined by the bubble behavior and the morphology of the metal film is determined by the nucleation and growth mechanism of the metal on the substrate.

Thus, two simultaneous reactions take place at the cathode, deposition of copper ions and the evolution of hydrogen bubbles. For electrolyte with copper sulfate, sulfuric acid and water, the reactions in electrolyte are given as follow.

At cathode: Cu²⁺+2e ⁻→Cu

2H⁺+2e ⁻→H₂(g)↑

At anode: Cu→Cu²⁺+2e ⁻

Electrolyte: CuSO₄→Cu²⁺+SO₄

H₂SO₄→2H⁺+SO₄ ²⁻

H₂O→H⁺+OH⁻

At higher applied currents, the electrode/solution interface is saturated with hydrogen bubbles that coalesce to form larger diameter bubbles. These bubbles reach a certain size, termed as “bubble break-off diameter” and detach from the surface to travel into the solution causing templating of the metals. The break off diameter is dependent on the bubble coalescence and their residence time which can be controlled by the applied current and duration.

Typically, the evolution of hydrogen bubbles can be controlled either by Galvanostatic (current dependent) or the Potentiostatic (potential dependent) method of deposition. Cauliflower type copper structures were observed at an overpotential of 550 mV without the evolution of hydrogen bubbles. The open and porous structures, such as honeycomb porous strictures are formed at overpotentials in the 800-1000 mV range. A Chronopotentiometric analysis of copper deposition reveals the evolution of hydrogen bubbles take place at a voltage greater than 0.84V. It is found by the methods known in the art that at the current density of 300 mA/cm², the atomic layer coverage occurs in the first step of the deposition process. Various researchers have implemented higher current densities in the range of from 400 mA/cm² to 2 A/cm² to create dynamic template assisted porous coating. The evolution of hydrogen bubbles is directly dependent on the concentration of the sulfuric acid in the electrolyte bath and hydrogen bubbles evolution and formation of porous structure is dependent on various factors such as temperature, duration, concentration of sulfuric acid, and distance between the electrodes. Additionally, some of the key parameters for the generation of the coatings include source of hydrogen ions and concentration of acid, over potential or current density, substrate material and its morphology.

Dynamic templating state: With increased current density or overpotential at which the electrolysis of water or hydrogen-based liquids happens, there is an abundant formation and cluster of hydrogen bubbles on the cathode surface which inhibit the continuous growth of the metal and the deposition of the metal occurs only within the interstitial spaces between the hydrogen gas bubbles. This state is dynamic templating state or dynamic hydrogen bubble templating state where hydrogen bubbles act as dynamic templates for the metal deposition and form the porous structure.

No-Dynamic templating state: Supply of current density or potential at which there is no electrolysis reaction leading to evolution of gas bubbles on the cathode surface in the desired area of interest (which is where coating is formed). This state is no-dynamic templating state or no-dynamic hydrogen bubble templating state where hydrogen bubbles do not act as dynamic templates for the metal deposition.

The advantages of the present technique over the existing electrodeposition techniques are as follows:

The bond strength of the electrodeposited coating with the substrate and between the coatings obtained from this technique is high.

The coating obtained is more durable. The high bond strength improves the durability. It endures aging in harsh environment. It survives repetitive boiling tests with little mechanical degradation. The coating has superior mechanical bond as compared to other electrodeposition techniques.

It is sustainable and reliable as compared to coating achieved through contemporary techniques.

The repetitive boiling performance of the proposed electrodeposited surface gives higher heat fluxes, reduces wall superheat temperatures at a given heat flux as compared to an uncoated surface and other electrodeposition techniques.

The surface degradation and/or corrosion is inhibited by coating a thin layer of metal that oxidize and present a barrier for degradation/corrosion of the underlying substrate. One example is to sputter coat aluminum on the electrodeposited surface and let it oxidize naturally. Other metals, other coating techniques and oxidization processes can be implemented. This technique could be combined with the multi-step electrodeposition technique to provide a strong bond as well as better aging characteristics of the coated surface. The electrodeposition process can be implemented in conjunction or as additional steps in other deposition or material reduction techniques such as sintering, 3D printing, machining, roughening, micromachining, chemical etching, etc.

Surface morphological features such as the size of the pores, porous network and the thickness of the deposited material can be tuned by varying the applied current density, deposition time and number of steps of the electrochemical process. These properties in turn govern the wettability and wickability possessed by the surfaces.

As per the requirement of the end result, the morphology including porosity, pore size and thickness of the coating can be controlled by varying the parameters of the electrodeposition process including, but not limited to, the current density (10 mA/cm² to 2500 mA/cm², more preferably between 10 mA/cm² and 1200 mA/cm²), voltage (2V to 100V or higher depending on the distance between the electrodes, more preferably between 2V and 30V), electrolyte composition (any suitable electrolyte containing salt ions capable of providing electrodeposition), choice of electrodes (metallic such as copper, gold, aluminum, titanium, platinum palladium, silver and their alloys; graphite and graphitic carbon; semiconductor, or any suitable electrode for electrodeposition process), spacing between the electrodes (1 mm to 25 mm, more preferably 3 mm to 10 mm), number of electrodeposition steps (3 or more, 4, 5, 6, 7, 8, 9, 10, preferably 3 to 10 or higher), these electrodeposition parameters in each step, and duration of each step (from 1 second to 1 hour or higher for a single step within a multi-step electrodeposition process, more preferably from 1 second to 2500 seconds for a single step within a multi-step electrodeposition process). The coating can be controlled to a thickness of from 5 μm to 500 μm, preferably from 10 μm to 100 μm. Some of the details are presented in FIGS. 2 and 3.

As per the requirement regarding the surface properties, the degree of hydrophilicity of the coatings, which is dependent on the porous networks of the deposited materials, can be controlled by varying the applied current density and its duration in different steps.

Due to additional increments in the number of steps in the electrodeposition process, the operating and controlling window is widened and hence a better control over the thickness and morphology of the coatings can be achieved.

The coating formation process is not just limited to Galvanostatic method, but the other deposition techniques such as chronopotentiometry, or chronoampometry can also be used for the deposition with applying multiple steps.

The substrate used for the coating can be of any metal, shape and thickness.

Substrate materials is not only limited to copper, but other materials such as silicon, aluminum, stainless steel, steel alloy, gold, silver, zinc, etc.

Carbon based nanomaterials, including but not limited to, graphene, carbon nanotubes, graphene sheets, and graphene nanoplatelets, and other metal composites can also be added to the electrolyte to form metal/carbon nanomaterial composite electrodeposited coatings.

Electronics cooling—the solution proposed here is very attractive in efficiently cooling of the high-power density electronics that is studied by evaluating pool boiling performance (quantified by heat transfer properties) over porous substrates that are produced by this technique. Specifically, the surfaces electrodeposited using this multi-step technique can be used to cool high heat flux electronics such as the CPUs of a server at data centers. The method can also be applied to electrochemical systems; corrosion and oxidation protected devices; reboilers in power plants; distillation units, nuclear plants, decorative systems with prolonged usage; chemical, petroleum and pharmaceutical equipment; brewery, food and beverage equipment; refrigeration systems and other applications where electrodeposited coatings are used.

Because of the better bond strength between the porous coating and the substrate, the coated surfaces can be effectively applied in many other applications, including but not limited to the lubrications to protect the wear resistance while retaining the lubricant or by ensuring a supply of the lubricant through a porous matrix. Controlling morphology while enhancing the surface properties will also be useful in providing corrosion resistance surfaces and the ability to tune surface properties surfaces where specific properties are desired while providing strong bond strength between the coating and the substrate.

Conventional electrodeposited surfaces deposited with existing electrodeposition techniques result in poor substrate bonds and may not withstand the forces exerted on the coating depending on the application. In one embodiment, the coating may peel off easily after the prolonged or continuous pool boiling test, while the coatings developed with the current technique withstand the forces from bubbles nucleating within the porous matrix and result in durable coatings. The aging performance may also be improved.

Implementation of the present multi-step electrodeposition technique results in more sustainable coating with improved substrate bonds and hence the test surface can be used repetitively for the pool boiling testing.

The aging properties of the electrodeposited surfaces are improved and thus the coating can sustain for longer duration. Aging refers to material changes that lead to performance degradation. This is also seen in the sustained performance following multiple or prolonged boiling tests and any other process being implemented on the coated surface, including physical wear, environmental and chemical degradation such as oxidation, etc.

Another novelty is that due to multi-step electrodeposition, the hydrophilicity of the coating can be modified which can in turn control the wicking properties of the coating thereby improving the pool boiling performance. Using the multiple steps, hierarchical structures with improved bond strength can be produced.

Dynamic template-assisted electrodeposition of porous coatings: template assisted electrodeposition allows precise control over the size and shape of the deposited materials. The templates could either be dynamic, restrictive or self-organized in nature. In case of aqueous solution, the electrolysis of water is one of the electrochemical reactions, the resultant evolved hydrogen gas bubbles serve as the dynamic template that results in porous surface coatings. Dynamic template-based coating used in the present invention, utilizes the co-evolution of hydrogen gas and the stripping of the cations from the metal electrode that is transferred through the electrolytic solution to create the porous structures by deposition and growing the metals around the generated bubbles on the substrate electrode that is to be coated. Upon deposition, the bubbles collapse leaving a porous network of the coated material on the surface. The rate of hydrogen bubble generation can be controlled by the applied current and the potential.

Electrodeposition modes of operation: electrodeposition apparatus is composed of electrochemical cell and power supply. Under Galvanostatic modes, the current between the electrodes is controlled, and under potentiostatic mode, the potential drop near the electrode surface is measured which is maintained by the presence of ions in the electrolyte. In the invention, Galvanostatic operation was used to maintain a constant current which was varied in a multi-step electrodeposition process. The process follows Faraday's law according to which, the amount of the deposited metal on the electrode is proportional to the applied current to the electrochemical cell.

$\begin{matrix} {Q = \frac{n*d*A*h*F}{M}} & (1) \end{matrix}$

Where, Q is the charge, n is no. of electrons, A is area of the coated surface, h is the height of the deposition, F is Faraday's constant, M is the atomic weight.

In this work, the duration of applied current is also taken into account.

Q=I*t  (2)

Where, q is the charge applied, I is the amount of current supplied in mA, and t is the duration for which the current has been supplied.

Adhesion and diffusion of the electrodeposition: the electrodeposited metal under the applied current forms a uniform and continuous layer on the substrate electrode as a result of interlocking of the grains of the deposited metals which further depends on the electric field distribution in the electrolyte. Hence, the composition of electrolyte, ionic species and their transport are additional factors in the formation of electrodeposited layer with tunable morphology as well as tunable wetting and wicking characteristics. The transport of ionic species in the electrolyte depends on several mechanisms including diffusion, migration, and convection. This embodiment relies on the diffusion process as the mode of transportation of the ions that form a gradient near the electrode surfaces. Redox reactions cause the depletion of the ions and their deposition on the substrate electrode via formation of an electric double layer. This electric double layer is composed of—Helmholtz, a monolayer deposition of ions on the electrode surface, and the Gouy-Chapman layer which represents the electrolyte outside the Helmholtz layer that is composed of the bulk of the electrolyte with ionic species. Outside, this double layer is the diffusion layer which is wider compared to the double layer and is formed as the result of change in concentration of ions during the deposition step. Other transport processes including migration of the ions under an applied electric field and convection resulting from the changes in concentrations and temperature also takes place in addition to the primary diffusion transport process. Therefore, the applied current and duration can be exploited to control the thickness of the coatings which can also be predicted by the following equation

$\begin{matrix} {h = \frac{M*Q}{n*d*A*F}} & (3) \end{matrix}$

where, h is the thickness of the deposition, M is the atomic weight, Q is the charge, n is no. of electrons, d is the density, A is the area and F is the Faraday's constant.

A multi-step electrodeposition technique includes multiple steps of varying applied current and duration are used while performing the electrodeposition process, each step having specific values of controlling parameters of the deposition. The electrodeposition technique can be performed on the substrate of any shape, size and material and its applications are not only limited to the test surfaces of boiling heat transfer, but also can be used for any electroplating and coating based applications including corrosion protection, decorative coatings, prolonged life of coating and surface, abrasion and wear resistance protection, durability, to maintain the aesthetics, integrated electronics, solar reactors, fabrications, and others. The multi-step technique is advantageous in number of ways, which are as follows:

The technique enables a better control over the pore dynamics such as size, network formation, distribution, resultant morphological features, and the thickness of the coating.

The technique helps to control the pore size and improve the cohesive and adhesive bond strength. The improved substrate bond strength makes the coating more sustainable and reliable.

The multi-step electrodeposition technique helps to alter the hydrophilicity and hydrophobicity of the resultant coating. The counter electrode material decides the nature of the coating formed and the hydrophilicity or the hydrophobicity can then be easily altered by varying the number of steps and parameters of the electrodeposition process.

This multi-step electrodeposition technique includes the enhanced surface properties and morphology and tunable deposition parameters (which are current density and deposition time in this particular application) and by varying the number of steps used in the electrodeposition process.

The multi-step electrodeposition process results in alternate deposition and growth of the coated layers. The deposition sites can be considered as the nucleation sites which are the islands of grains of the depositing metal ions that further form compact deposits that stimulate the coating growth on the surface of the substrate electrode. The thickness of the deposited layer depends on the clusters of ions deposited on the surface, the concentration of which depends on the current.

In an embodiment, an electrochemical cell used for the multi-step electrodeposition of metal on metal, involves both working electrode and counter electrode of copper. An electrolyte solution includes CuSO₄, conc. H₂SO₄, and distilled water. A six-step electrodeposition is performed on the plain copper chip.

An embodiment of the three-step electrodeposition process starts with a supply of low current density for a longer period to deposit the base layer of copper followed by the supply of high current density for a shorter period with the simultaneous evolution of hydrogen bubbles to deposit copper and finally a supply of low current density for a longer period to strengthen the deposition. For the six-step electrodeposition of copper, a smaller duration of the high current density is divided into three short durations followed by the supply of low current density for a longer period after each of the three short durations. This strengthens the cohesive and adhesive bonds of the deposition. The detailed process of six-step electrodeposition technique is explained below.

The Table 1 below shows the steps involved for the six-step electrodeposition.

Step # Current density (mA/cm²) Time (sec.) Step 1 High Current (HC-1) Lower Time (LT-1) Step 2 Low Current (LC-1) Higher Time (HT-1) Step 3 High Current (HC-2) Lower Time (LT-2) Step 4 Low Current (LC-2) Higher Time (HT-2) Step 5 High Current (HC-3) Lower Time (LT-3) Step 6 Low Current (LC-3) Higher Time (HT-3)

The control of porosity and morphology can be obtained with the six-step electrodeposition technique. As shown in Table 1, for the first step, when high current HC-1 is supplied, the copper deposition occurs with the evolution of hydrogen bubbles. The hydrogen bubbles on the copper surface assist in developing the porous structure. The growth, separation and coalescence of these hydrogen bubbles determine the pore size and the density. Once the high current density supply is stopped, the evolution of hydrogen bubbles also terminates, and the porous structure is formed.

A low current density is applied in the second step. This impedes the evolution of hydrogen bubbles at the cathode. The higher duration HT-1 in the second step facilitates the bonding of coating with the base material. In HT-1, shells of the deposited coatings are formed, and the interconnection of the coating is performed. This way the bond of the deposition is enhanced due to the 2^(nd) step.

In the third step, when HC-2 is applied, again, there is an evolution of hydrogen bubbles, which starts building another layer of the deposition. The evolution of hydrogen bubbles takes place over the entire cross section of the test chip. This means that bubbles will form at the already deposited coating and as well as in the voids which were previously formed as shown in FIG. 3A. This phenomenon helps in reducing the size of the pores by simultaneously depositing the copper. The new layer starts forming and it enhances the bonding with the existing coating on the surface (deposited due to first two steps of the electrodeposition). Moreover, the second-high current step HC-2 also helps to enhance the bonding between the deposited material in the voids and the base material.

After application of this 3^(rd) step, the 4^(th) step of longer duration begins. In this HT-2 step, the activity of strengthening of the deposition of newly deposited coating of the copper is performed. The lower current and the longer time insures the slow and steady deposition rate and the enhancement of the adhesive bonding. Due to reduced rate of evolution of hydrogen bubbles, the bonding of the deposition with the existing coating is improved.

When the 5^(th) step of high current density is applied again, as shown in FIG. 3B, the new layer of the deposition starts building again. This step will help in refining the pore size and morphology of the structure will be controlled. The application of final 6^(th) step of the deposition will again assist in improving the bonding and thus the enhanced structure with controlled morphology will be achieved.

The total number of steps can be more or less than the 6-step process described above depending on specific morphological or applications requirements.

Effect on surface properties due to the deposition technique:

By the multi-step electrodeposition technique, a better control over the wettability and the wickability can also be obtained. Due to enhanced morphology and the porosity, the static, the advancing and the receding contact angles are decreased and thus the contact angle hysteresis. This multi-step electrodeposition technique improves the wicking properties such as wicking rate, and wicking number of the deposited structure. However, as per the requirement of the surface properties, by reducing the growth and the coalescence of the hydrogen bubbles, and by reducing the pore size, the hydrophilic or hydrophobic nature of the coating can be controlled.

Various Alternate Surface Modulations:

The pore diameter can be increased by increasing the deposition time of the High Current step (i.e., HC-1, HC-2, and/or HC-3). Pore diameter is the function of deposition time of the high current density. Higher duration will ensure the growth and the coalescence of the hydrogen bubbles. Depending upon the requirement of the end result of the porosity, the current density and the time in each step of high current densities i.e. the HC steps can be varied. However, a very high time duration might cover the entire test surface with the hydrogen bubbles and can eventually prevent the deposition of the coating to the base material. One of the important advantages of this multi-step electrodeposition technique is the control over the pore size, diameter of the pores, and pore density. The existing techniques of the electrodeposition do not have the same degree of control over porosity and all the other properties mentioned above.

Similarly, the shorter duration of the high current density steps (i.e., HC-1, HC-2, and/or HC-3) will ensure the formation of the small pores across the surface. The shorter duration can help in increasing the number of pores by decreasing the size of the pores. Thus, the control over the pore size and density is obtained in each of the high current steps (HC-1, HC-2, and/or HC-3).

The longer duration of the lower current density steps (i.e., LC-1, LC-2, and/or LC-3) will help in strengthening the substrate bonding of the coating. As per the requirement of the end result, the duration and the current density of the lower current steps can be varied to achieve the enhanced bonding. The current during any step may further be varied to yield the desired morphological properties and bond strength.

Apart from this, by varying the electrochemical method of deposition, the multi-step technique can be employed to the respective method of deposition and can have a controlled morphological structure.

As the number of steps in the electrodeposition technique increase, the uniformity of the coating increases and after 9 steps, a solid thick layer of the metal or the anode material gets deposited. In a particular application of boiling heat transfer, the thickness of the deposited coating also acts as a thermal insulation layer and hence, in this particular application maximum 7 steps have been used for the electrodeposition to limit the overall thickness.

Techniques used for surface characterization:

The substrate bond strength of the deposition is conventionally measured using the scotch tape test. In this experimentation, apart from tape test, the boiling test is considered as a measure of the bond strength. The bond strength will be established by conducting prolonged boiling tests and seeing the performance degradation in the boiling curve over the repetitive testing. Also, the sample will be inspected through visual inspection and Laser Confocal Microscope (LCM) to detect the changes. If the bond strength is low, then this will result in degradation in boiling performance during repetitive testing and the surface morphological changes will be detected.

Bond strength is the ability of the coating to remain stick to the substrate or the base surface.

Scanning Electron Microscope (SEM) images are captured to capture the surface morphologies and elucidate the surface properties such as pore size, pore diameter, and surface height.

Morphology is defined as the physical surface features on the substrate that shows a network of hierarchical pores formed as a result of electrodeposition.

Goniometer is used to determine various contact angles (static, advancing, and receding) of the electrodeposited test surfaces and to find the wickability of the surfaces. Hydrophilicity of the surfaces is determined by wicking rate and the contact angles.

Wickability of the surface is defined as the ability of the surface to absorb a particular volume of the droplet. It is the measure of porosity and hydrophilicity. Higher the wicking rate, higher is the porosity and hydrophilicity.

Hydrophilic surfaces possess strong affinity for water.

Higher wicking rate and/or wicked volume of the surfaces result of higher porous and hydrophilic surfaces. A higher wicking rate is also indicative of better interconnection among the neighboring pores.

In another embodiment, other materials are added to the electrolyte such as graphene, graphene oxide, reduced graphene oxide, graphene nanoplatelets (GNP), single walled and/or multi-walled carbon nanotubes, and any metal or non-metal substance and/or colloidal solution alone or in combination with other substances may be added to the electrolyte. This results in composite material electrodeposited coatings. Such electrolytes are then used for generated electroplated coatings and result in complex coated structures in which different materials are embedded in the electrodeposited material. These composite coating surfaces will possess different tunable properties and morphologies which may be controlled by the electrodeposition process conditions. These may be deposited by 3-step, 4-step or multi-step electrodeposition technique.

The disclosure will be further illustrated with reference to the following specific examples. It is understood that these examples are given by way of illustration and are not meant to limit the disclosure or the claims to follow.

EXAMPLES

The long-term use and the substrate bond strength are the critical issues for enhancing pool boiling performance on the electrodeposited surfaces. In one of the embodiments, a multi-step electrodeposition technique is developed to enhance the substrate bond strength of the coating and the long-term use of the test surfaces. In this multi-step technique, an alternate activity of deposition and strengthening the deposition is performed to build a structure with enhanced bonding. The test surfaces coated with this technique were then experimentally investigated for the pool boiling performance with water at atmospheric pressure. To ensure the long-term enhancement, the repetitive pool boiling tests were performed on the multi-step electrodeposited surfaces and the performance was compared with the existing two-step electrodeposited test surface. For the multi-step electrodeposition of copper on copper, the CHF of 213 W/cm² was achieved for seven-step electrodeposition at the wall superheat of 21.6° C., which is ˜42% higher as compared to two-step electrodeposited test surface. CHF enhancement of ˜72% and ˜93% enhancement in Heat Transfer Coefficient (HTC) was achieved as compared to plain copper chip. Also, the multi-step electrodeposited chips performed better in long term/aging studies as compared to the chips deposited with existing electrodeposition technique. The improvement in porosity due to multi-step electrodeposition increased the nucleation sites thus improved CHF and reduced wall superheat. Enhanced wicking rate was also responsible for the enhancements in pool boiling performance.

Some of the highlights of this work include:

Pool boiling performance of degassed water was studied.

A critical heat flux (CHF) of 213 W/cm² at the wall superheat of 21.6° C. was achieved for the copper on copper electrodeposited surface on the plain copper chip.

High wicking rate was responsible for improved boiling performance.

Due to enhanced substrate bonding, the aging performance was improved.

The growing need of electronics systems to dissipate the larger heat has received a considerable amount of attention since a past four decades. Significant research in this field started after the first application in electronics cooling. It is known from the methods in the art that the convective heat transfer coefficient between the substrate and the coolant is the primary impediment to achieving low thermal resistance. To improve the pool boiling performance further, various surface enhancement techniques have been implemented, some of which include microchannels, pins or fins, or cavities.

Pool boiling performance of the micro porous surfaces has been reported extensively by the methods known in the art. Porous surfaces have higher heat flux at low wall superheat due to increased surface area and nucleation sites. To be effectively tested for the pool boiling testing, bond strength of the porous surface must be strong enough such that the coating should not come off. Steady vapor formation takes place on the porous media and the nucleation takes place within the matrix via the re-entrant cavities that are not susceptible to flooding by liquid. When the heat is supplied to the heater surface, nucleus of a bubble grows in the cavity. The cavities may be formed inside the porous matrix or may appear on the surface of the coating. When this bubble nucleates, it carries heat with itself. As the bubble departs, the liquid in the vicinity of void fills the cavity, thus there is continuous supply of fluid for the evaporation. Higher the nucleation frequency, higher the heat dissipated from the surface. Further, the agitation caused by the bubble activity may increase the heat transfer rate between the surface and the liquid.

Research has indicated that the use of electrodeposition enhances the boiling performance. The porous size of the surface increases with increase in time of the deposition. Also, the porosity is increased with increase in evolution of hydrogen bubbles and can be controlled by controlling the percentage of concentrated sulfuric acid. But, the deposited structure comes off during the pool boiling tests as reported by the methods known in the art and hence bonding is the important issue to get the stable results. A technique is proposed in which deposition is obtained by initially coating at higher current densities for a short duration, and lower current densities for a longer duration. In the first step, deposition of copper along with simultaneous evolution of hydrogen bubbles occurs, leaving behind porous copper. In the second step, the current density is taken such that the copper is deposited without evolution of hydrogen. This improves the bonding and the porosity of the copper surface. Hence, this method is used to create the micro porous surfaces.

Several ways to form porous surfaces for boiling heat transfer are investigated by three main manufacturing techniques that are commonly used for creating porous boiling surfaces—sintering, electrodeposition, and advanced techniques such as vapor blasting, jet impingement, spray coating. Critical heat flux of 325 W/cm² at a wall superheat of 7.3° C. has been achieved for a surface with porous microchannel fin tops using two-step electrodeposition.

Test Section:

Plain test chips of made of copper alloy 101 were used in this boiling study. Test chips consisted of a 17 mm×17 mm×9 mm surface with a 9-mm deep rectangular base 1 to accommodate the thermocouples used for heat flux estimation. The actual test area under consideration is 10 mm×10 mm 6. The thermocouple holes were drilled to reach the center of the rectangular base. As shown in FIG. 4, the distance between the two successive holes on a rectangular base is 3 mm (Ax) while the distance between the hole near the chip surface and the top of the chip is 1.5 mm. (x₁). To read the three temperatures, T₁ 2, T₂ 3, T₃ 5, thermocouples were inserted into the holes. FIG. 5 is the photographic image of the test surface showing the delineated working area 7.

Electrodeposition:

The experimental procedure of the boiling study consists of two parts, the first involving the electrodeposition of copper on copper, and then the testing of the boiling performance of the depositions. A copper substrate, also known as the chip, is machined in house from a copper 101 (Cu) alloy to be used in the deposition phase. The chip will act as the working electrode with a copper block as the counter electrode to create the electrochemical cell. FIG. 6 shows a schematic of the electrochemical cell that is used for the deposition process. The working area of the chip will be a 10 mm×10 mm square area and is polished in order to maintain an even surface and therefore to maintain even current density supply. The counter electrode 42, i.e., copper block, is also polished. A Polytetrafluoroethylene (PTFE) holder 48 was designed to maintain the working 29 and counter electrode parallel to each other. The distance of 3 mm between the cathode and the anode is maintained for all the electrodeposition processes. The working area was delineated with Kapton® tape as shown in FIG. 5. Once both working and counter electrodes are placed in the holder and connected to the power source 26 via insulated copper wires 38, the whole structure is placed in an electrolyte solution 33. Solution consisted of 5.85 gm of 0.8M CuSO₄, 3.14 mL of 1.5M conc. H₂SO₄, and 40 mL distilled water. Since the chemical reaction of CuSO₄ and H₂SO₄ is exothermic, water is added in two intervals, 20 mL before adding H₂SO₄ and 20 mL after adding H₂SO₄.

The Galvanostatic method of deposition which is a constant current method, was used for the deposition of copper. The electrochemical deposition was divided into two parts; first was the deposition of copper on copper with traditional two-step method and the second was the deposition of copper on copper with new multi-step electrodeposition.

Example 1: Prior Art Two-Step Electrodeposition Technique

In two-step electrodeposition technique, different current densities are applied in each step with different time of the deposition. For the creation of micro-porous surface, evolution of hydrogen bubbles is essential. Optimum current density and the time were chosen for the copper on copper deposition. The two-step deposition was performed by initially coating with higher current density for a shorter duration of time, followed by the lower current density for longer duration of time.

In the first, deposition of copper along with evolution of hydrogen bubbles occurred, while in the second step, lower current density was chosen such that deposition of copper occurred without evolution of hydrogen bubbles. For the step 1, current density of 400 mA/cm² is applied for 15 seconds, while for the second step; a current density of 40 mA/cm² is applied for 2500 seconds.

Example 2: Six-Step Electrodeposition Technique

It was noted that, after the pool boiling test, coating formed by the two-step electrodeposition technique had peeled off. This showed that the bonding of the deposited coating with the base material is not strong. Hence, in order to improve the bonding, there is a need of new electrodeposition technique that can sustain for a longer duration and does not come off the base material. Also, it is estimated that, due to enhanced bonding, the new electrodeposition technique will improvize the pool boiling performance of the electrodeposited copper chip.

Hence, a multi-step electrodeposition technique is developed for improving the bonding. This new electrodeposition technique involves an alternate activity of the deposition and strengthening the deposition to build the more robust structure. The new electrodeposition technique uses the same principle as that of the two-step electrodeposition. But, the formation of deposited layer is done in smaller increments. This allows the strengthening of the deposition. A two-step electrodeposition is divided into six-step electrodeposition, which involves an alternate activity of the deposition and strengthening of the deposition.

Similar to two-step deposition, the six-step electrodeposition technique uses higher current density for short duration with simultaneous deposition of copper and evolution of hydrogen bubbles, and small current density for longer duration without evolution of hydrogen bubbles. But, instead of supplying a large current continuously for 15 sec., it is divided in 3 steps followed by a small current for longer period after each small step of large current density. The overall time of higher current density is kept constant but is divided in the intervals of 5 sec. Table 2 below shows the steps involved in six-step electrodeposition.

TABLE 2 Six-step electrodeposition technique. Current density Step # Step ID (mA/cm²) Time (sec.) 1 HC/LT 400 5 2 LC/HT 40 2500 3 HC/LT 400 5 4 LC/HT 40 2500 5 HC/LT 400 5 6 LC/HT 40 2500

Example 3: Three-Step Electrodeposition Technique

Another possible variation includes the electrodeposition activity in three steps. If the electrodeposition is performed by starting with the low current density for long duration step, the evolution of hydrogen bubbles will be very less and this step will give the foundation or base to the next electrodeposition steps. This will improve both the substrate bond strength and the cohesive bond strength. For the electrodeposition process, similar values as mentioned in previous techniques were implemented. Table 3 below shows the steps involved in three-step electrodeposition.

TABLE 3 Three-step electrodeposition technique. Current density Step # Step ID (mA/cm²) Time (sec.) 1 LC/HT 40 2500 2 HC/LT 400 15 3 LC/HT 40 2500

Example 4: Seven-Step Electrodeposition Technique

To improvise the six-step electrodeposition technique further, another electrodeposition activity with variation of total number of steps, current density and time is performed. The deposition was started with the small current density with longer duration step, and an alternate activity of high and low current steps was followed to perform the deposition. Table 4 below shows the number of steps and respective current and time values. The low current density with longer duration provides the foundation for the high current density step in which the rapid evolution of hydrogen bubbles is observed. This gives a drastic improvement in the substrate and deposition bond strength.

TABLE 4 Seven-step electrodeposition technique. Current density Step # Step ID (mA/cm²) Time (sec.) 1 LC/HT 40 2500 2 HC/LT 400 5 3 LC/HT 40 2500 4 HC/LT 400 5 5 LC/HT 40 2500 6 HC/LT 400 5 7 LC/HT 40 2500

Example 5: Pool Boiling Experiments

In this example, the pool boiling test setup used for the experimentation is explained. The experimental set-up for the pool boiling part of the procedure is shown in FIG. 7. The bottom garolite plate 47 consisted of a ceramic chip holder 45 to hold the heater surface 36 over which a quartz glass water bath 13 measuring 14 mm×14 mm×38 mm is assembled by means of 4 stainless steel socket head cap screws (¼″ diameter). A glass water bath is used chiefly to aid visualization and is sealed to the ceramic chip holder by means of a rubber gasket 64 which covers the area outside the boiling surface. A middle plastic plate 17 holds the water bath on the upper side and is connected to the top aluminum plate 31 by means of 2 stainless steel socket head cap screws (¼″ diameter) 42. A water reservoir is mounted between the middle plastic plate and top aluminum plate. The water reservoir is sealed with rubber gaskets on either side to ensure against leakage at all times. The top aluminum plate is provided with two circular openings for the saturation thermocouple probe and a 60-VDC, 200 W auxiliary cartridge heater 56 to maintain water in the reservoir at saturation by boiling continuously.

The bottom section of the setup consists of a 120-VDC, 4×200 W capacity cartridge heater 49 inserted into a copper heater block. The copper block consisted of a truncated portion measuring 10 mm×10 mm×40 mm that fits into the groove on the bottom side of the ceramic chip holder. This ensured that 10 mm×10 mm surface of the heater is in contact with the test chip 36 which also has a base section measuring 10 mm×10 mm which facilitated 1D conduction from the heater to the test chip. Additionally, the copper block is housed on a ceramic sleeve to minimize heat losses. Four compression springs 61 supported the bottom aluminum plate 34 that provided the required degree of movement to establish contact between the test chip and the heater and accommodate for any expansion during the testing. A shaft pin (⅜″ diameter) connected the middle garolite plate, bottom garolite plate and the work desk which ensured stability of the setup.

Data Acquisition:

To record the temperatures given by the thermocouples, a National Instruments cDaq-9172 data acquisition system with NI-9211 temperature module was used. Overall four thermocouples were used, out of which three were inserted into the slots provided in the test section for heat flux measurement and surface temperature determination, while the fourth was inserted from the top of aluminum block to measure the saturated temperature. A LabVIEW VR virtual instrument is displayed, and the surface temperature and heat flux were calculated as shown in FIG. 8. Data acquisition unit 77 is used to read the temperatures and display it in LabVIEW. Grafoil sheet 41 between the copper heater 53 and test chip 36 assists in reducing the air resistance. Cartridge heaters 49 conduct the heat through the chip which boils the water bath 57. Apart from that, Lab View also shows the graphical variation of temperature with respect to time of each thermocouple. This is useful for determining the critical heat flux spikes.

Heat flux is calculated using steady state 1D conduction equation

$\begin{matrix} {q^{''} = {{- k_{Cu}}\frac{dT}{dx}}} & (4) \end{matrix}$

Where, the temperature gradient dT/dx was calculated using the three-point backward Taylor's series approximation

$\begin{matrix} {\frac{dT}{dx} = \frac{{3T_{1}} - {4T_{2}} + T_{3}}{2\Delta \; x}} & (5) \end{matrix}$

The boiling surface temperature was obtained by using eq. 1 and 2, and is given by

$\begin{matrix} {T_{wall} = {T_{1} - {q^{''}\; \left( \frac{x_{1}}{k_{Cu}} \right)}}} & (6) \end{matrix}$

Example 6: Uncertainty Analysis

In this example, the uncertainty analysis performed on the data is explained. During the experiment, a certain amount of error occurs in measuring the values. The two main errors are precision error and bias error. Bias errors are the difference between expected measurement value and the true measurement value. So, bias errors are the errors due to calibration while precision errors are due to the sensitivity of the testing instruments. A complete uncertainty analysis was performed similar to Patil and Kandlikar. Errors due to precision and bias uncertainty are expressed as:

U _(y)=√{square root over (B _(y) ² +P _(y) ²)}  (7)

Where, U_(y) is the uncertainty of parameter y. P_(y) is the precision error and B_(y) is the bias error. Table 5 shows the parameters associated with precision errors and the source of these errors, which includes thermocouple temperatures, thermal conductivity of copper and the machining of test chip. The thermocouples are calibrated, and its precision error was computed statistically to be ±0.1° C.

TABLE 5 Uncertainty parameters and sources of error. Precision error Parameter Value Units (U_(P)) % uncertainty T_(TOP) Varies ° C. 0.0763 Varies T_(MIDDLE) Varies ° C. 0.0756 Varies T_(BOTTOM) Varies ° C. 0.0791 Varies k_(Cu) 391 W/m° C. 9 2 Δx 3.00E−03 M 1.00E−04 3 Δx₁ 1.50E−03 M 1.00E−04 6

$\begin{matrix} {U_{p} = \sqrt{\sum\limits_{i = 1}^{n}\left( {\frac{\partial p}{\partial a}*u_{ai}} \right)^{2}}} & (8) \end{matrix}$

Where U_(p) is the uncertainty in the parameter p, and u_(ai) is the uncertainty of measured parameter a_(i). The uncertainty in the heat flux and heat transfer coefficient can thus be expressed by the equations 9 and 10 respectively.

$\begin{matrix} {\frac{U_{q^{''}}}{q^{''}} = \sqrt{\begin{bmatrix} {\left( \frac{U_{k}}{k} \right)^{2} + \left( \frac{3U_{T_{1\;}}*k_{Cu}}{\Delta \; x*q} \right)^{2} +} \\ {\left( \frac{4U_{T_{2}}*k_{Cu}}{\Delta \; x*q^{''\;}} \right)^{2} + \left( \frac{U_{T_{3}}*k_{Cu}}{\Delta \; x*q^{''}} \right)^{2} + \left( \frac{U_{\Delta \; x}}{\Delta \; x} \right)^{2}} \end{bmatrix}}} & (9) \\ {\frac{U_{h}}{h} = \sqrt{{\frac{U_{q^{''}}^{2}}{q^{''\; 2}} + \frac{U_{T_{w}}^{2}}{\Delta \; T_{sat}^{2}} + \frac{U_{T_{sat}}^{2}}{\Delta \; T_{sat}^{2}}}\;}} & (10) \end{matrix}$

FIG. 9 shows the uncertainty in heat flux for six-step electrodeposited chip. It was observed that uncertainty decreases with increase in heat flux. The main aim of this study is to have the uncertainty below 5% at higher heat fluxes and at CHF. Similar calculations were done to find the uncertainty in HTC (h). The uncertainty was found to be below 5% in the same region. Uncertainty analysis was performed for all the test chips.

Example 7: Characterization of the Electrodeposited Surfaces

Scanning Electron Microscope (SEM).

In this study, as mentioned above, two different electrodeposition techniques were used to deposit the copper on copper. The distinct morphological structures are analysed using a JSM-6400V and TESCAN Field Emission Mira III Scanning Electron Microscope (SEM), at an accelerating voltage of 15 kV. The Energy Dispersive X-ray Spectroscopy (EDS) measurements were done on Bruker Quantax EDS with XFLASH 5010 detector attached to a field emission scanning electron microscope MIRA II LMH to the presence of various elements on the sample.

Comparison of 2-S and 6-S copper on copper chips: As shown in FIG. 10 (c), SEM image of the two-step deposited chip appears as a cauliflower type structure. A different morphological structure is obtained with a six-step electrodeposition technique.

FIG. 11 shows SEM images of two and six-step copper on copper electrodeposited chips at a magnification of 10 kX. A porous structure with minimal pore spacing, and openness is obtained. However, the six-step deposited chip has formed a highly micro porous structure with improved bonding and improved porosity.

Observations:

From FIG. 11 a), it is observed that cauliflower type structure is obtained for 2-S technique. The diameter of the structure formed using 2-S electrodeposition technique is in the range of 5 μm to 12 μm. However, the diameter of the structure formed using 6-S electrodeposition technique is in the range of 0.8 μm to 5 μm. Thus, more intricate structure with the less feature diameter is achieved using the newly proposed 6-S technique.

The pores available on the structure deposited with 2-S deposition are very less and have the diameter range of 3-5 μm. On the other side, highly porous structure is obtained using 6-S electrodeposition technique. A wide range of pore size is achieved using this deposition technique. The pore diameter obtained has the range from 0.75 μm up to 5 μm. In another embodiment, the pore diameters may range from 0.5 μm to 20 μm and 0.5 μm to 50 μm. The pore size is highly dependent on the dynamic templating process parameters such as current density, duration, type of material. Metals with higher interactions with hydrogen, high current, shorter duration will lead to an increased bubble generation, which can lead to advanced porous network formation. However, higher pore sizes, larger than about 20 μm may lead to a weaker bound coating unsuitable for industrial applications.

From the SEM, it can be concluded that by using six-step (6-S) electrodeposition technique, better control on morphology, porosity, pore density, and the pore size, as well as the feature size is obtained. This control of the parameters mentioned above is not possible just by using the 2-S deposition. Also, compared to 2-S deposition technique, a wide range of feature size, and pore diameter is gained using 6-S technique and thus achieving the better morphological structure.

Comparison of 2-S and 3-S Copper on Copper Chips:

FIG. 12 compares the SEM images of 2-S and 3-S electrodeposited chips. As seen from the FIG. 12 b), the feature size of the 3-S chip is higher than 2-S chip. The cauliflower structure is more prominent in 3-S chip. Apart from that, the pore size of 3-S chip has become smaller which further can help in improving boiling performance.

Observations: Compared to 2-S, 3-S developed prominent cauliflower structure having diameter ranging from 3 μm to 16 μm. the feature size for 2-S deposited chip is in the range of 5 μm to 12 μm. The pore size and density achieved using 3-S technique is higher, ranging the pore size from 0.9 μm to 4 μm. Porosity achieved using 2-S technique is lower and cannot be controlled. Hence, the proposed technique is advantageous in terms of improving the morphological properties of the coatings.

Comparison of 2-S and 7-S Copper on Copper Chips:

FIG. 13 compares the SEM images of 2-S and 7-S electrodeposited chips. As seen from the FIG. 13 b), the feature size of the 7-S chip is higher than 2-S chip. The cauliflower structure is more prominent in 7-S chip. Apart from that, the pore size of 7-S chip has become smaller which further can help in improving boiling performance.

Observations: As shown in FIGS. 13 b), d), unique morphology is achieved using 7-S electrodeposition technique. the cauliflower structure is merged and stacked to form the morphology having larger range of feature size and increased porosity. A wide range of feature size from 2 μm to 15 μm is achieved and this structure is achieved at different levels of deposition as seen in FIG. 13 b). Top and bottom layers can be distinguished from the SEM images and pore size obtained is in the range of 0.5 μm to 5 μm. As the number of steps increase, the wicking rate increases indicating that the underlying substrate has more interconnected pores. This morphology promotes boiling heat transfer performance by supplying liquid from surrounding areas to the nucleation sites.

End of Example 7

Metal Oxide Protective Layer Over the Substrate.

The oxidation and decrease in durability of the copper substrates as a result off from multiple boiling tests and aggressive solutions can be extended by depositing protective coatings or films onto these surfaces. One of the simplest methods of obtaining a high quality controlled and well adhered thin coating is sputter coating technique that involves use of a glow discharge to strike a target of the desired film concentration with energetic ions as a result the target atoms are dislodged from the surface, or sputtered, and gets deposited on the substrate. A variety of materials that have high mechanical strength, corrosion and wear resistance such as carbides, nitrides, metal oxides and other transition metals can be used as coating materials. The controlling parameters in sputter coating process are—sputtering voltage, current, duration, and rise in temperature. The copper substrates will be sputter coated with aluminum layer allowed to oxide naturally in the atmosphere. This oxidized aluminum coating will offer enhanced thermal and chemical properties. The deposition rate and microstructure can be controlled by increasing the power density and gas flow rates.

Example 8: X-Ray Diffraction

In this example, implemented x-ray diffraction technique is discussed. X-ray diffraction is a technique used for determining the atomic and molecular structure of a crystal in which crystalline atoms cause a beam of X-rays to diffract in specific directions. When X-rays are incident on the sample, incident beam get separated in transmitted beam and diffracted beam. The diffraction pattern is recorded in term of 2θ angle that indicates the crystalline phase of the material. The crystalline phases of the copper coated substrates were investigated using a Rigaku DMAZ-IIB X-Ray Diffractometer (XRD) with Cu Kα radiation; wavelength 1.5418 Å. The spectra were recorded for 2θ ranges between 5° and 75° at a rate of 3°/min rate. The step size was 0.02° with an X-ray power of 40 kV and 35 mA. This range is expected to capture peaks from carbon and the underlying copper substrate. The location of characteristic peaks determines the presence of elements on the surface. XRD was performed on the test chips to ensure the deposition of copper on the test surfaces. As shown in FIG. 14, the peaks at 40°, 47°, and 54° indicate the presence of copper on the electrodeposited chips.

Example 9: Contact Angle Study

Static, advancing and receding contact angles were measured on the test surfaces. Contact angle measurements were done using VCA Optima Goniometer Instrument. Measurements of the contact angles were recorded on the VCA software which showed the real-time visualization of the droplet. For calculating the contact angle, five points were used to trace the shape of the drop and then left side and the right-side contact angle of the droplet was displayed.

Static contact angle was measured by dropping the specific quantity (2 μL) of the droplet of water on the surface and stabilizing it for 5 seconds. For measuring the advancing contact angle, syringe was used to drop a certain volume of water (4 μL) on the surface. The angle was measured when the size of the droplet was increased and when the droplet of water slide outward on surface compared to its previous position. The maximum value of the contact angle was recorded as the advancing contact angle. Similarly, for measuring the receding contact angle, water from the chip surface was drawn into the syringe, and the point at which the water droplet changed its shape from convex to concave, the contact angle was measured. This was recorded as the receding contact angle. The difference between advancing and receding contact angle is called as the hysteresis of contact angle.

TABLE 6 Comparison of various contact angles for different electrodeposition technique. Contact angle (°) Test Chip Static Advancing Receding Hysteresis Cu on Cu (Two-Step 53 67 18.4 48.6 deposition) (2-S) Cu on Cu (Three-Step 58 65 21 44 deposition) (3-S) Cu on Cu (Six-Step 26 29.6 9.9 19.7 deposition) (6-S) Cu on Cu (Seven-Step 32 38 12.2 25.8 deposition) (7-S)

Observations:

As seen from the Table 6, the static, advancing, and receding contact angle values for 2-S deposited chip are much higher as compared to multi-step electrodeposited chips. Lesser the contact angle, higher is the hydrophilicity of the surface. Hence, by implementing newly proposed electrodeposition technique, the surface becomes more hydrophilic. Thus, achieving lower contact angles and higher hydrophilicity is not possible by using existing electrodeposition techniques.

By using proposed multi-step electrodeposition technique, better control over contact angle and hydrophilicity can be easily achieved. Table 6 shows the improved hydrophilicity obtained using multi-step electrodeposition process.

Wicking Rate.

Wicking is the property of micro porous structure which measured the volume of droplet wicked or absorbed in a particular period. For measuring the wicking rate, fixed volume of water (2 μL) was dropped on the chip surface and dynamic contact angle data of the droplet was taken. A sessile drop method was used to measure the wicking rate. A pendant water droplet was slowly brought contact to the electrodeposited test surfaces. Using high speed camera, rate of change of volume of droplet was measured on a software on frame basis. The wicking rates of each surface was measured by visualizing the dynamic spreading behavior of a drop placed on these surfaces. The volumetric change in a liquid droplet of a fixed volume was captured for a duration of ˜75 s using a VCA Optima goniometer.

Comparison of 2-S and 6-S: As shown in FIG. 15, for the time frame of ˜80 seconds, contact angle reduction for six step (6-S) deposited chip is much higher as compared to two step (2-S) deposited chip. Contact angle for 6-S chip reduced from 45° to approximately 16° within 80 seconds showing that wicking rate for 6-S is way higher than 2-S. FIG. 15 shows the wicked volume for both 2-S and 6-S deposited copper on copper chips. It is observed that wicked volume rate for 6-S chip marginally exceeds as compared to the 2-S chip.

Thus, by using 6-S electrodeposition technique, drastic improvement in wicking rate and wickability is achieved which is not achievable using 2-S or any other electrodeposition techniques. FIG. 16 shows the wicked volume over the period of 80 seconds for 2-S and 6-S chips. From the plot, it is observed that the wicked volume for 6-S is very high, thus it shows that the porosity and the wickability of 6-S chip is very high. Thus, the proposed multi-step electrodeposition technique helps in improving both porosity and the wicking properties of the surface.

Comparison of 2-S and 7-S: Wicking rate was calculated for the 7-S chip using similar technique as mentioned above. It was observed that the wicking rate and the wicked volume for the 7-S was higher than the 2-S chip. This again confirms the improved porosity and the wicking properties of the chip due to implementation of new multi-step electrodeposition technique. FIGS. 15, 16 show the wicking rate and wicked volume comparison of 2-S and 7-S. FIG. 17 compares the change in contact angle over a period of time for copper coatings deposited with 2-step and 7-step process. FIG. 18 presents the change in droplet volume over a period of time on copper coatings produced using 2-step and 6-steps electrodeposition methods.

The thicknesses of the electrodeposited coatings were measured using Laser Confocal Microscope. It was observed that the coating thickness increases with increase in number of steps during the electrodeposition. As shown in FIG. 19, due to the application of Kapton tape 69 on the sides during electrodeposition, the difference between the heights of the coating 85 and pristine copper surface 73 could be easily measured. The Kapton tape is removed after electrodeposition and thus difference in height could be easily measured. The thickness of each electrodeposited chip was measured at four edges of the deposition with considering the average step height achieved and the average value was taken as the final thickness of the coating. The increment rate of thickness of coating however, reduces with increase in number of steps.

Table 7 below shows the thickness of the coating after each deposition step. The increment in thickness reduces as the number of steps increase.

TABLE 7 Average coating thicknesses Average thickness of coating Chip (μm) 2-S 43 3-S 64 4-S 72 6-S 96 7-S 117

Example 10: Results and Discussions on Boiling Performance

Results section consists of two different sections. The pool boiling curve is plotted for all the electrodeposited chips deposited with new multi-step techniques and the results are compared with the copper on copper two-step electrodeposition and with the plain copper chip. In the next section, comparison of the aging study of the multi-step electrodeposited chips and two-step electrodeposited chip is performed.

CHF and HTC enhancement due to new electrodeposition techniques.

FIG. 20 shows the pool boiling curves obtained with the 2-S, 3-S, 6-S, and 7-S copper on copper electrodeposited chips. It is observed that the CHF of the chips deposited with the proposed multi-step electrodeposition techniques is much higher than the existing two-step electrodeposited chip and a plain copper chip. CHF of 213 W/cm² at wall superheat of 21.6° C. was achieved for the seven-step electrodeposited chip. For the copper on copper electrodeposition on a plain surface, this is the highest heat flux that has ever been achieved. Compared to plain copper chip, ˜72% enhancement in CHF and 93% enhancement in HTC was achieved with 7-S chip. For the 6-S chip CHF of 192 W/cm² was achieved at wall superheat of 19° C. The 3-S deposited chip also gave higher performance than 2-S chip, giving CHF of 169 W/cm² at wall superheat of 15.6° C.

FIG. 21 shows the comparison of heat transfer performance of the different electrodeposited chips. It is observed that 7-S chip gave the CHF of 213 W/cm², giving 72% enhancement in CHF than that of the plain copper chip. From the Scanning Electron Microscope (SEM) images shown in the Characterization section, it was noted that the improvement in pore size is advantageous for enhancing the boiling performance. Improved morphological structure increased nucleation sites, which helped in rapid boiling process and increased CHF by simultaneously reducing wall superheat. Table 8 below shows the comparison of CHF and HTC of various chips with plain copper chip.

TABLE 8 CHF & HTC comparison with respect to plain copper chip. % % CHF enhancement HTC (kW/m²- enhancement Chip (W/cm²) in CHF ° C.) in HTC Plain Cu chip 124 52 2-S 151 22 66 27 3-S 170 37 109 108 6-S 192 55 101 94 7-S 213 72 100 93

Hence, the proposed multi-step electrodeposition technique improves the boiling performance drastically and also improves the efficiency of the proposed 6-S electrodeposition technique shows significant improvement in boiling performance and thus is preferred than 2-S deposited chip.

Example 11: Test Procedure of Aging Study

Due to continuous pool boiling testing, morphology of the test surface gets affected which results in reduction in boiling performance of the chip and the coating removal. The test surface created using the proposed multi-step electrodeposition technique aims at improving the long-term use of surface by altering the morphology of the surface and by improving the porosity.

To compare the advantages of the newly employed electrodeposition technique in terms of longevity and adhesion improvement, aging study was carried out. Boiling involves vigorous motion of bubbles on the surface and causes damage to the electrodeposited surface. The damage is more if the substrate bond strength is less and if the adhesive forces are weak. These weak forces also reflect in the reduction of pool boiling performance and increase in wall superheat temperature. Wall superheat temperature is the temperature difference between substrate surface temperature and the saturation temperature of the bulk fluid. Hence, if the bond strength is more, the boiling performance will be better and wall superheat will be lower.

In this study, the bond strength will be established through the boiling studies by conducting repetitive boiling tests over long durations and observing the performance degradation in the boiling curve over the repetitive testing. Also, the degradation of the sample will be inspected through visual inspection and Laser Confocal Microscope. If the bond strength is low, then this will result in degradation in boiling performance during repetitive testing.

The repetitive pool boiling tests are performed on the same chip to age the electrodeposited surfaces. Heat flux is raised up to 80 W/cm² for each pool boiling test and chip is kept running at the same heat flux for 15 minutes, and then the heat flux is again reduced back to zero, thus tracing the curve backwards. Between two successive pool boiling tests, the test surface undergoes the cooling cycle of 24 hours. After the repetitive testing, the same chip is then tested till it reach the critical heat flux (CHF). The CHF performance of the fresh chip and the aged chip is then compared.

Results: Comparison of Aging Study of 2-S and 6-S Chips:

At the end of 3^(rd) repetitive test, it was observed that the test surface of 2-S deposited chip had started to damage, and the coating had come off partially. But, after performing 7 repetitive pool boiling tests, the coating of the two-step (2-S) deposited chip was destroyed completely. However, the coating of six-step (6-S) deposited chip was not reduced at all. Hence, it can be said that the new multi-step electrodeposited chip has better bonding and the adhesion of the deposition to the surface is very strong.

FIG. 22 shows the photographic images of the surfaces of the aged chips after 7 repetitive tests. It is evident from FIG. 22A that the coating of the surface of two step 51 deposited chip has completely come off. Compared to that, coating of the six-step 79 deposited chip is still present, indicating that bonding is better for six step deposited chip. FIG. 23 shows the comparison of Laser Confocal Microscope images of 2-S and 6-S chips after the 7^(th) repetitive tests. From FIG. 23A it was confirmed that the surface of the 2-S deposited chip 62 has been destroyed due to repetitive boiling tests. This inspection indicates that the bonding of the 2-S electrodeposited chip is weak, and the surface is degraded due to continuous activity of boiling. However, compared to 2-S, as indicated in FIG. 23B, the conformation of coating on 6-S chip has been shown 66. Hence the repetitive boiling test is used to establish the bond strength of the electrodeposited surfaces. FIGS. 24, 25 show the comparison of wall superheat at different repetitive tests that indicates that for the same heat flux of ˜80 W/cm², wall superheat of six-step electrodeposited chip i.e. 6-S, is significantly less than that of the two-step (2-S) deposited chip.

For the 3^(rd) repetitive test, as shown in the FIG. 25, for the same heat flux, wall superheat of 2-S chip is 17.6° C. while that of the 6-S chip is 14.3° C. Again, the similar trend is observed in all the tests. (4R to 7R). Maximum of 60 kW/m²° C. HTC was obtained for 6-S chip at the 3^(rd) repetitive test, while 44 kW/m²° C. HTC was obtained for the 2-S chip at 3^(rd) repetitive test. It is observed that, for a given heat flux, the HTC of SS chip is always higher than 2-S chip. This indicates that Six-Step (6-S) deposited chip has higher heat transfer efficiency than that of the Two-Step (2-S) deposited chip.

Effect of Aging on CHF and HTC.

As explained earlier, due to peeling off the coating of 2-S chip, the CHF obtained for 2-S is not accurate. FIG. 26 shows the CHF of the aged chips. Compared to first CHF value, aged chips have less CHF. Wall superheat of 2-S chip is increased as well. However, for the 6-S chip, wall superheat temperature has remained similar as that of mentioned above. CHF of 152 W/cm² was obtained for the aged 6-S chip at wall superheat of 18.8° C., while CHF of 140 W/cm² at a wall superheat of 23° C. was obtained for 2-S chip.

HTC of 84 W/m²° C. was achieved for 6-S chip at the CHF, while for the 2-S chip, it was 61 W/m²° C. Compared to HTC of fresh samples, HTC and CHF of the aged chips reduced. However, newly employed Six-Step electrodeposited chip performed better than two-step deposited chip. CHF, wall superheat and hence HTC of the 6-S chip is higher than 2-S indicating the improvement in performance due to new electrodeposition technique as shown in FIG. 27.

Comparison of Aging Study of 3-S, and 7-S Chips:

As mentioned above, similar studies were performed on chips 3-S and 7-S. However, the repetition was kept limited to just three times. No characterization was involved and only repetitive pool boiling tests were performed on these chips. Lower wall superheat will indicate the improvement in boiling performance and thus will confirm that the 3-S and 7-S techniques will have better substrate bonding and both strength will be better.

FIG. 28 shows the comparison of wall superheat of 3-S deposited chip at different repetitive tests. At the end of 3^(rd) repetitive test, the wall superheat of the 3-S chip was observed to be 14.9° C. This temperature is less than the wall superheat of 2-S chip at the end of the third repetitive test. For the 2-S chip, wall superheat was 17.6° C., and compared to that, wall superheat of 3-S is lesser. This shows that aging performance of 3-S is better and thus has a higher bond strength.

FIG. 29 shows the repetitive boiling performance of 7-S copper on copper electrodeposited chip. After the 3^(rd) repetitive test, it was observed that for the heat flux of 80 W/cm², the wall superheat was just 13.4° C. Compared to this, the observed wall superheat of 2-S was 17.6° C., and of 6-S was 14.3° C. This is the lowest wall superheat achieved for aging study of the multi-step electrodeposited chips. Thus, the 7-S chip is the best performing chip in terms of aging and thus has the highest substrate bond strength. FIG. 30 represents a comparison of wall superheat studies performed by repeating pool boiling tests. R represents the repetition run number. The data represented are for the tests conducted on 117 μm thick coatings obtained via 2-step electrodeposition method.

Example 12: Thickness Study

In order to support the fact that the enhancement achieved in CHF and in the aging of test surface is not just because of higher thickness but is mainly due to the proposed multi-step electrodeposition technique, a plain copper chip with the thickness of ˜117 μm (same as 7-S electrodeposited chip, refer Table No. 7) was electrodeposited using the two-step electrodeposition technique. To achieve this thickness, for the step 1, current density of 650 mA/cm² was applied for 35 seconds, while for the second step; a current density of 60 mA/cm² was applied for 7000 seconds. The Laser Confocal Microscope was used to confirm the achieved thickness of the coating. In addition to this, after performing aging study with 3 repetitive pool boiling tests, thickness of the aged 2-S chip (deposited with new parameters mentioned above) and of aged 7-S chip was measured. If the measured thickness of the coating of aged 2-S chip is less than that of the aged 7-S chip, it will ascertain that the substrate bond strength and adhesion of the coating of 2-S chip is less than that of the 7-S chip.

Similar test procedure of pool boiling experiments (as explained in test procedure of aging study section) of the chip was followed for the aging of 2-S chip. FIG. 32 below shows the repetitive pool boiling curve of 2-S chip of ˜117 μm thickness. It was observed that, with increase in number of repetitions of tests, the wall superheat went on increasing. At the end of the 3^(rd) repetitive test, wall superheat of 14.8° C. was observed for 2-S chip. Compared to 7-S chip, which had a wall superheat of 13.5° C. after 3^(rd) repetitive test, the wall superheat of 2-S chip with same thickness as that of 7-S chip is much higher.

Apart from that, after 3′ repetitive test, thickness of the 2-S chip was measured as 91 μm, while that of the 7-S chip was 108 μm. The higher thickness of the 7-S chip indicates that despite of repetitive pool boiling tests, due to strong adhesion and higher bond strength the thickness didn't reduce much. This proves that, compared to conventional electrodeposition techniques, the present multi-step electrodeposition technique provides a very high bond strength and adhesion to the substrate.

End of Example 12

The substrate bond strength of the electrodeposited coating obtained from this technique is high.

The coating obtained is more durable. It endures aging in harsh environment. It survives repetitive boiling tests with little mechanical degradation. The coating has superior mechanical bond as compared to other electrolytic deposition techniques.

The repetitive boiling performance of the proposed electrodeposited surface gives higher heat fluxes with reduced wall superheat temperatures as compared to a plain surface and other electrolytic deposition techniques.

Better control on morphology is achieved using the controlling parameters such as current density and the deposition time using this proposed technique.

As per the requirement of the end result, the porosity, pore size and the thickness of the coating can be controlled by controlling electrodeposition process parameters including, but not limited to, the current density (10 mA/cm² to 2500 mA/cm², more preferably between 10 mA/cm² and 1200 mA/cm²), voltage (2V to 100V or higher depending on the distance between the electrodes, more preferably between 2V and 30V), electrolyte composition (any suitable bath capable of providing electrolytic deposition), choice of electrodes (metallic, graphitic, semiconductor, or any suitable electrode for electrodeposition process), spacing between the electrodes (1 mm to 25 mm, more preferably 3 mm to 10 mm), number of electrodeposition steps (from 2 to 10 or higher, more preferably 3 to 10 or higher), and the electrodeposition parameters in each step, and duration of each step (from 1 second to 1 hour or higher for a single step within a multi-step electrodeposition process, more preferably from 1 second to 2500 seconds for a single step within a multi-step electrodeposition process).

The coating formation process is not just limited to Galvanostatic method, but the other deposition techniques such as chronopotentiometry, or chronoampometry can also be used for the deposition with applying multiple steps.

It is possible to apply two low current density steps in succession with each step having different electrodeposition parameters.

It is possible to apply two high current density steps in succession with each step having different electrodeposition parameters.

Although various embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the disclosure and these are therefore considered to be within the scope of the disclosure as defined in the claims which follow. 

1. A multi-step electrodeposition method comprising: providing an electrolyte comprising positive and negative ions; placing an anode electrode and a metal substrate cathode electrode in the electrolyte; and implementing a multi-step process between the anode electrode and the cathode electrode, wherein the multi-step process comprises three or more steps of alternating a low current density step sufficient to produce deposition of a metal layer but insufficient to produce dynamic templating deposition of metal on the substrate and a high current density step sufficient to produce dynamic templating deposition of metal on the substrate.
 2. The method of claim 1, wherein the multi-step method comprises: a first step comprising a low current density for long duration without evolution of hydrogen bubbles supporting dynamic templating deposition to provide a base layer to support the following deposition; a second step comprising a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles and dynamic templating deposition; and a third step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 3. The method of claim 2, wherein the multi-step method further comprises: a fourth step comprising a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and a fifth step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 4. The method of claim 3, wherein the multi-step method further comprises: a sixth step comprising a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and a seventh step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 5. The method of claim 1, wherein the multi-step method comprises: a first step comprising a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition; a second step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition; a third step comprising a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition; and a fourth step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 6. The method of claim 5, wherein the multi-step method further comprises: a fifth step comprising a high current density for a short duration with deposition of metal and simultaneous evolution of hydrogen bubbles supporting dynamic templating deposition; and a sixth step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 7. The method of claim 6, wherein the multi-step method further comprises: a seventh step comprising a high current density for a short duration with simultaneous deposition of metal and evolution of hydrogen bubbles supporting dynamic templating deposition; and an eighth step comprising a low current density for long duration with deposition of metal without evolution of hydrogen bubbles supporting dynamic templating deposition.
 8. The method of claim 1, wherein the metal comprises copper, aluminum, titanium, platinum, gold, palladium, silver and alloys thereof; graphite; or graphitic carbon.
 9. The method of claim 1, wherein the current density comprises from 10 to 2500 mA/cm² at a voltage from 2V to 100 V.
 10. The method of claim 1, wherein the electrolyte composition comprises CuSO₄, H₂SO₄ or H₂O.
 11. The method of claim 1, wherein the multi-step process deposits a total layer thickness comprises from 5 to 500 μm.
 12. The method of claim 1, wherein the multi-step process deposits a total layer thickness comprises from 10 to 100 μm. 